Cultured human cell lines derived from malignant tumors when injected subcutaneously in nude, thymus-deficient mice produce, in the great majority of cases, malignant neoplasms. Some, however, do not grow. Human cells cultured from non-neoplastic tissues never grow as tumors when injected subcutaneously in nude mice. Some, however, grow as such if injected intracerebrally and in newborn nude mice, but not if injected intracerebrally or in newborn non-nude mice. Such cells grow also subcutaneously in adult nude mice further immunosuppressed with antilymphocyte serum. We do have now available a large colony of nude mice and smaller colonies of nude mice with additional genetic immunological defects: 1) C3H nude mice (partial B-deficiency) 2) Lasat mice (congenitally asplenic); 3) N;NIH(F)-2-NU (total B-deficiency). The objective of the present research is to find under which condition of immune suppression which cultured cells acquire "malignant potential", i.e. the ability of growing indefinitely in the host and of invading its tissues. The cells to be so tested were all incapable of growth when injected subcutaneously in adult nude mice. They will be injected intracerebrally in adult nude and non-nude mice, in newborn nude and non-nude mice, subcutaneously in adult mice, further immunosuppressed by total body radiation or by injection of antilymphocyte serum. They will also be injected subcutaneously into groups of nude mice genetically further immunosuppressed (Lasat-C3H-N;NIH(F)-2-NU). The growth of each line will be correlated with the specific type of immunosuppression necessary to allow for its growth.